The Development of High Accuracy Methods for the Measurement of Metal Speciation
The copper binding protein rusticyanin (Rc) was used as a model system to investigate the potential for performing species-specific isotope dilution mass spectrometry (ssIDMS) analysis of a protein. A ®^Cu enriched copper binding Rc spike was prepared from Rc produced using a bacterial recombinant system. The stability of the ®^Cu binding spike was investigated using both inductively coupled plasma mass spectrometry (ICP-MS) and electrospray ionisation mass spectrometry (ESI-MS). The spike was shown to be stable. An oyster metallothionein (MT) was also produced using recombinant techniques. Methylmercury (MeHg) was determined in an International Measurement Evaluation Programme (IMEP) laboratory intercomparison tuna fish test sample, IMEP-20 tuna material. External calibration using high performance liquid chromatography inductively coupled plasma mass spectrometry (HPLC-ICP-MS) produced a MeHg (as compound) dry weight mass fraction of 4.26 ± 0.53 pg g’^ (coverage factor, K = 2). Details of the uncertainty budget formulated are also presented. Extract storage for 6 months resulted in a reduction in the MeHg mass fraction by 14-16 %. compared to the original MeHg levels. Both ssIDMS and species-unspecific isotope dilution mass spectrometry (sulDMS) calibration modes were used to determine MeHg by HPLC-ICP-MS in IMEP-20. For ssIDMS calibration a dry weight mass fraction of MeHg (as Hg) in IMEP-20 tuna material was found to be 3760 ± 66 ng g’^ (K = 2), giving quantitative recovery. An uncertainty budget is also presented The measured calibration blend isotope amount ratio (R’bc) and the measured sample blend isotope amount ratio (R’B) were found to make dominant contributions to the measurement uncertainty. However, for sulDMS calibration, a dry weight mass fraction of MeHg (as Hg) in IMEP-20 tuna material was found to be 1.98 ± 0.075 ug g-1 (± 1 SD), showing that equilibrium had not been reached as only 50 % recovery was achieved.
Networked Digital Library of Theses and Dissertations